This invention relates to particulate matter classifiers. More particularly, the present invention relates to apparatus for separating particles of a certain fineness from larger particles.
It has long been known in the prior art to provide apparatus for purposes of effecting the grinding or pulverizing of certain materials. More specifically, the prior art is replete with examples of various types of apparatus that have been used to effect such grinding of a multiplicity of materials. In this regard, in many instances discernible differences of a structural nature can be found to exist between individual ones of the aforesaid apparatus. The existence of such differences is, in turn, attributable for the most part to the diverse functional requirements that are associated with the specific applications in which such apparatus are designed to be employed. For instance, in the selection of the particular type of apparatus that is to be utilized for a specific application one of the principal factors to which consideration must be given is that of the nature of the material that is to be ground in the application. Another factor to which consideration must be given is that of the fineness to which it is desired to grind the material.
Turning next to a consideration of separator devices, as the name given thereto implies, the function of a separator device is to effectuate in some preestablished fashion a separation of material which is made to enter the separator device. Apparatus have long been known to be available in the prior art which are suitable for use for such a purpose.
One such separator apparatus that is a mechanical air separator or classifier. One application in which particular use has been made of mechanical air separators is that relating to the grinding and classifying of cement wherein a mechanical air separator device has been combined with a grinding device so as to form a closed circuit therewith. When operated in closed circuit combination with a grinding device, the mechanical air separator is designed to skim off the fines as fast as they are produced such that the grinding device works only on fresh material without wasting power. The tailings from the mechanical air separator, however, are discharged back to the grinding device for further reduction. After being reground in the grinding device, the reground material is returned to the mechanical air separator along with the feed that is being supplied thereto so that a constant circulating load is established between the grinding device and the mechanical air separator. Mechanical air separators are also commonly employed in closed circuit grinding operations for producing limestone sand to meet close specifications of granular sand material to be used in bituminous concrete, mortar, as an aggregate and many other uses; for making fine, uniform cake mixes and for the production of protein-enriched grades of flour; for producing a high fineness, uniformly classified, hydrated lime for chemical and spray purposes; for classifying numerous food products including sugar, cocoa, milk powder, food mixtures with various ingredients, corn starch and wheat starch, and soya bean meal; in applications where manufactured chemicals are required in closely sized form, i.e., for making the various grades ranging from extremely fine to the granular dust free gradations of such chemicals as soda ash and sodium phosphate; in the beneficiation of certain materials such as talc, kaolin and clays, and phosphate rock for purposes of removing therefrom impurities in the form of silica, flint and other foreign materials; for classifying metal powders consisting of copper, bronze, iron and various alloys and for de-dusting of seacoal for foundry facing use, etc.
As regards the matter of efficiency, the higher the efficiency of a mechanical air separator, the closer the fractional recovery comes to 100% at the finest particle sizes. A mechanical air separator's inability to attain 100% fractional recovery is referred to as bypassing. More specifically, bypassing is defined as being the difference between a numerical value of 100% and the amount of fractional recovery that is actually attained at the finest particle sizes. Bypassing is believed to be caused by one or more of the following three events. One of these is the internal recirculation of fines. A second is the inadequate dispersion of the feed in the air prior to the feed reaching the classifying zone of the mechanical air separator. The third is the existence of an excessive material/air ratio which has the effect of causing interference between particles within the classifying zone of the mechanical air separator.
In a variation utilized for the production of pulverized coal that is to be burned as fuel in a coal fired power generation system, the mechanical air separator is housed within the coal pulverizing apparatus. In particular, separation of pulverized material occurs as a consequence of causing the air within which the pulverized material is entrained to follow a tortuous path through the mechanical air separator, whereby in the course of changing directions of flow the larger of the particles of the pulverized material lose their momentum and are made to return to the surface of the grinding table whereat they are subjected to further pulverization. The means by which this separation is generally accomplished is by way of a static classifier, as shown in U.S. Pat. No. 5,873,156 for example, or a rotary classifier, as shown in U.S. Pat. No. 5,657,877 for example.
In a static classifier, the flow of primary air and coal particles entrained therein is directed through a series of stationary turning vanes which make up the aforesaid convoluted path. Said turning vanes are canted at an angle to the direction of the flow of the stream of primary air and coal particles so as to cause the coarsest (and therefore heaviest particles) to fall out of the primary air stream and return to the grinding table to suffer a second pulverizing action.
In a rotary classifier, the flow of primary air and coal particles entrained therein is directed through a series of vanes disposed as an inverted, truncated cone and revolving about the central vertical axis of the housing at a predetermined rotational velocity in a squirrel cage fashion. The vanes are canted at an angle to the direction of the flow of the stream of primary air and coal particles entrained therein so as to present to the stream a window through which the stream of primary air and coal particles may pass unimpeded. However, the rotational velocity of the vanes coupled with the velocity of the primary air stream and the coal particles entrained therein acts to separate the coal particles into two groups. A first group of particles are those that are relatively coarse or heavy and therefore unable to pass unimpeded through the aforesaid window and are thus returned to the grinding table to suffer a second pulverizing action. A second group of particles are those that are relatively fine or light and therefore able to pass unimpeded through the window and thus be directed through the remainder of the bowl mill and delivered to the furnace of the steam generator.
For a fixed velocity of the primary air stream, by the judicious manipulation and control of the aforesaid rotational velocity of the vanes, the relative fineness of the two groups of coal particles may be adjusted, i.e., by increasing the rotational velocity of the vanes, the fineness of the coal particles that pass through the aforesaid window increases. In other words only increasingly finer particles will pass unimpeded as rotational velocity increases whereas increasingly coarser coal particles will pass unimpeded as rotational velocity is reduced. Conversely, for a fixed rotational velocity of the vanes, by the judicious manipulation and control of the aforesaid velocity of the primary air stream, the relative fineness of the two groups of coal particles may be adjusted, i.e., by increasing the velocity of the primary air stream, the fineness of the coal particles that pass through the aforesaid window decreases. In other words coarser and coarser particles will pass unimpeded as primary air velocity increases and finer and finer coal particles will pass unimpeded as primary air velocity is reduced.